High frequency component



Oct. 16, 1962 "r. s. SAAD 3,059,201

HIGH FREQUENCY COMPONENT Filed Feb. 11, 1959 FIG.2 F\G.3

F l G. 4 INVENTOR.

THEODORES-SAAD ATTORNEYS United States Patent 3,059,201 HIGH FREQUENCY COMPONENT Theodore S. Saad, Westwood, Mass., assignor to Sage Laboratories, Inc., Wellesley, Mass. Filed Feb. 11, 1959, Ser. No. 792,590 4 Claims. (Cl. 338-416) The present invention relates in general to high frequency circuit components and more particularly concerns a novel resistive element adapted to reside within a wave transmission conduit and which presents a constant impedance to energy transmitted through the con- (hit over a wide frequency range. The invention may serve as a matched load or as a precision attenuator having an input and output impedance which matches that of the wave transmission conduit.

The novel resistive element is especially useful as an attenuator in a coaxial transmission line. A typical prior art coaxial attenuator generally included resistive material place along the center conductor of the transmission line for an axial distance related to the degree of attenuation desired. For single frequency operation, such an attenuator may be designed to perform satisfactorily. However, it is diflicult to obtain a precise degree of attenuation for a wide range of frequencies, particularly at higher microwave frequencies, because the length of the resistance-coated center conductor is generally not small compared to a wavelength. Such devices become more frequency sensitive as the degree of attenuation desired increases, because a greater length of resistive coating is required.

Accordingly the present invention contemplates and has as an important object, the provision of a coaxial attenuator for providing a prescribed degree of attenuation over a relatively wide range of frequencies while matching the characteristic impedance of the coaxial transmission line at both the input and output end faces of the attenuator.

It is another object of the invention to provide a high frequency resistive element adapted to reside within a coaxial transmission line and function as a virtually reflectionless load over a wide range of frequencies.

It is a further object of the invention to provide high frequency resistive elements in accordance with the preceding objects which are compact, inexpensive, easy to insert into a coaxial transmission line and capable of dissipating power at relatively high levels.

According to the invention, the novel resistive element comprises a solid cylinder of lossy material having its circumferential surface adapted to contact the inner surface of the outer conducting wall of a wave transmission conduit when inserted therein. At least, one end face of the cylinder is formed with at least one groove, the dimensions and location of the grooves and the length of the cylinder being selected so that the characteristic impedance of the wave transmission conduit is matched at the end face when the element is located within the wave conduit. When used in a coaxial transmission line, the end face is formed with a central portion adapted to contact the center conductor of the coaxial transmission line and at least one anular groove is concentric about this central portion. For use as an attenuator, both end faces of the cylinder are formed with such grooves so that both the input and output of the attenuator thus formed presents an impedance which matches that of the wave transmission conduit. The length of the cylinder determines the degree of attenuation.

.Other features, objects and advantages of the invention will become apparent from the following specification when read in connection with the accompanying drawings, in which:

FIG. 1 is a perspective view of the novel resistive elements suitable for use in a coaxial transmission line;

FIG. 2 is a diametrical sectional view of a coaxial transmission line with the element of FIG. 1 located therein;

(FIG. 3 is a modification of the structure shown in FIG. 2, wherein the resistive element rests within a recessed groove in the inner surface of the coaxial transmission line outer conductor, and;

FIG. 4 is a schematic circuit diagram of an analogous low frequency resistive circuit.

With reference now to the drawings and more particularly FIG. 1 thereof, there is shown a perspective view of the novel high frequency element 11 suitable for use in a coaxial transmission line. The element is pref erably a unitary structure with the circumferential surface 12 coated with a layer of conductive material to insure good contact with the inner surface of the outer conductor of the coaxial transmission line. The end face 13 is formed with a raised central portion or hub 14, surrounded by a coaxial annular groove 15. An outer annular shoulder portion 1 6 completes the formation of the end face 13. When used as an attenuator, the opposite end face 17 is formed in the same manner as face 13.

Referring to FIG. 2, the element 11 is shown located within a coaxial transmission line. The center hub 21, concentric annular groove 22 and concentric annular shoulder 23 of end face 17 correspond to similar parts 14, 15 and 16, respectively of end face 13. The coaxial transmission line includes an outer conductor 24, an inner conductor 25, whose end is in contact with hub 14 and a center conductor 26 in contact with hub 21.

Element 11 forms an attenuator which matches the characteristic impedance of the transmission line at both end face 13 and end face 17. The degree of attenuation is proportional to the volume of element 11. The attenuation may be increased by lengthening element 11. The axial depth and radial width of grooves 15 and 22 are selected so that the coaxial transmission line is matched at both the input and output end by faces of the attenuator.

Referring to FIG. 3, there is shown an alternate way of locating the element within a coaxial transmission line. The outer conductor 24 is formed with a recessed annular groove in its inner surface for accommodating the annular shoulder portion 23. This is advantageous from the standpoint of mechanical rigidity since the inner conductors 25 and 26 are not required to support the attenuating element 11. This minimizes the possibility that the center conductors might buckle and cause a disturbance in the symmetry of the electro-magnetic fields within the transmission line.

Referring now to FIG. 4, the mode of operation of the novel resistive element will he better understood by considering an analogous low frequency circuit, such as the resistive pi network shown in FIG. 4. Since in most practical applications, the input and output impedance of the attenuating element should match the same line characteristic impedance, the equivalent pi network is symmetrical. Thus looking in at the input end, 1-1 of the element, it is desired to see a conductance of G the characteristic impedance of the transmission line with the output terminals '22 loaded by the characteristic impedance G of the output section of transmission line, as indicated. In a pi network, the transfer admittance is entirely determined by the value of the conductance G the transfer admittance being the ratio of the current conducted through a short circuit across terminals 22 to the voltage applied at terminals 1--'1. Increased attenuation may be obtained 'by decreasing the value of the conductance 3 G This may be accomplished by increasing the length of element 11. However the input admittance seen at terminals 1-1 is given by Therefore, when the attenuation is increased by decreasing G the shunt admittance G should be increased so that the input admittance seen at terminals 11 remains substantially constant. The value of the shunt admittance G may be controlled with a negligible effect on G by varying the dimensions of the groove 15 and 22. As the depth or radial width of this groove is increased, radial electric field penetrates less lossy material at the face 13 between the center and outer conductor of the coaxial line and shunt admittance G is elfectively increased. Conversely, reducing these dimensions reduces the value of G. It is of interest to note that as G decreases, corresponding to an increase in the length of the cylinder 11, the shunt admittance G should increase so that the depth of the groove may be nearly always made small compared to the total axial length of the attenuating element 1 1. It is preferred that the depth of these grooves remain small compared to a wave length of the highest frequency energy propagated through the coaxial transmission line. This prevents the degree of attenuation from being frequency sensitive.

From the description of the mode of operation, it is evident that the novel resistive element may also be used as a terminating load resistance for a coaxial transmission line. When used as a terminating load, it is generally necessary to form a groove only in the face nearest the propagated energy.

There has been described a novel high frequency resistive element especially suitable for use in a coaxial transmission line as an attenuator or load impedance.

In a representative embodiment of the invention, generally of the shape shown in FIG. 1 With the circumferential surface 12 coated With silver, an attenuation of 18 db+ or .5 db over a frequency range from 4000 to 8000 me. was obtained with an element 11 .240" long, .440" in diameter with the outer and inner diameters of grooves and 22 being .270 and .120, respectively to an axial depth of .097, when located in a 50 ohm. coaxial transmission line with the VSWR never exceeding 1.29 within the indicated frequency range.

It is evident that those skilled in the art may now make numerous modifications of and departures from the specitic embodiments described herein, without departing from the inventive concepts. Consequently, the invention is to be construed as limited only by the spirit and scope of the appended claims.

What is claimed is:

1. A resistive element adapted for insertion into a coaxial transmission line capable of propagating high frequency energy comprising, a cylinder of lossy material with a solid cylindrical portion thereof surrounding the axis thereof and having its circumferential surface coated with conducting material, said conducting material being adapted to contact the inner surface of the outer conducting Wall of said transmission line, and having at least one end face formed with at least one groove therein, the orientation and dimensions of said groove and the length of said cylinder being selected to match the characteristic impedance of said transmission line at said one end face with said circumferential surface in contact with said inner surface.

2. A resistive element for insertion into a coaxial transmission line comprising, a cylinder of lossy material with a solid cylindrical portion thereof surrounding the axis thereof and Whose circumferential surface is coated with conducting material, said conductively coated surface being adapted to contact the inner surface of the outer conductor of said coaxial transmission line and formed With an end face having a central portion adapted to contact the center conductor of said coaxial transmission line, said end face having at least one annular groove concentric about said central portion, the dimensions and radial position of said groove and the axial length of said cylinder being such that said element matches the characteristic impedance of said coaxial transmission line when located therein with said central portion in contact with said center conductor and said circumferential surface in contact with said outer conductor inner surface.

3. High frequency apparatus comprising, a coaxial transmission line having an inner and outer conductor, a cylinder of lossy material with its circumferential surface in conductive contact with the inner surface of said outer conductor, said cylinder being formed with at least one end face having at least one groove therein, the orientation and dimensions of said groove and the length of said cylinder 'being selected to match the characteristic impedance of said transmission line at said one end face with said circumferential surface in contact With said inner surface.

4. High frequency apparatus comprising, a coaxial transmission line having a center conductor and an outer conductor, a cylinder of lossy material whose circumferential surface is in conductive contact with the inner surface of said outer conductor, said cylinder being formed with an end face having a central portion in conductive contact with said center conductor, said end face having at least one annular groove concentric about said central portion, the dimensions and radial position of said groove and the axial length of said cylinder being such that said element matches the characteristic impedance of said coaxial transmission line.

References Qited in the file of this patent UNITED STATES PATENTS 2,557,122 Leiph'art June 19, 1951 2,620,396 Johnson Dec. 2, 1952 2,831,163 Stevens Apr. 15, 1958 

